The United States and Canada generate about 100,000,000 pounds of spent base-metal catalyst per year, about half of which is spent hydroprocessing catalysts. The present invention relates to a process for regenerating spent, heavy hydrocarbon, hydroprocessing catalysts. More specifically, the present invention relates to a process for regenerating spent, heavy hydrocarbon, hydroprocessing catalysts that have been deactivated with coke and metal deactivants such as nickel and vanadium.
With respect to the present invention, the term hydroprocessing is used to refer to a process for hydrodemetallation, hydrodesulfurization, hydrodenitrogenation, and hydroconversion wherein the term hydroconversion encompasses the hydrocracking and hydrotreating of hydrocarbon streams containing asphaltenes and contaminant metals. Hydroprocessing catalysts used to treat heavy hydrocarbon streams, such as resids, are deactivated as a result of metals deposition and coke deposition. These deposition materials modify the rate of reaction as well as accelerate the rate of catalyst deactivation. The various metal deposits tend to occlude catalyst pores and poison the hydroprocessing catalyst, while coke deposits similarly reduce pore size and surface area of the hydroprocessing catalyst.
Typically, hydroprocessing catalysts possess substantial macropore volume in order to effect metals removal from heavy hydrocarbon feed streams. Heavy hydrocarbon, hydroprocessing catalysts possess the capacity to adsorb contaminant metals, such as nickel and vanadium, in an amount ranging up to about 100 wt. % of the fresh catalyst weight. However, due to the rapid coke deposition rate, the catalyst is deactivated prior to achieving its full metals adsorption capacity. Such catalyst are taken out of service when they contain as little as 10 wt. % nickel plus vanadium.
Thus, the prior art is replete with processes suitable for handling these spent catalysts. These processes can be separated into four areas: (1) disposal, (2) total catalyst reclamation, (3) catalyst rejuvenation, and (4) catalyst regeneration.
Disposal of spent catalyst requires compliance with stringent environmental standards that can substantially increase the cost of handling spent catalyst. One acceptable method of disposal of spent catalyst is encapsulation, wherein the catalyst is surrounded, either as a pellet or in bulk, with an impervious layer of sealant. Bitumen, paraffin wax and polyethylene have been used as thermoplastic encapsulating agents in which the dry waste is mixed with the sealant at high temperature and cooled. Similar methods have been developed for low temperature encapsulation, where polybutadiene binder can be mixed with the catalyst followed by application of a thin polyethylene jacket around the mass. Although encapsulation provides a relatively effective means of disposing of spent catalyst, the possibility of fire and weathering can lead to long term instability of the encapsulants. In addition, care must be taken to avoid shear stresses that can break open encapsulated particles.
Total catalyst reclamation involves removing both the contaminant and catalytic metals from the spent catalyst to recover as precious metals. The use of catalyst as a source of precious metals is well known in the industry. Most metal recovery methods involve roasting the spent catalyst with or without the presence of additives, followed by leaching of the catalyst. A suitable method of recovering precious metals from spent catalyst is presented in G. Parkinson, "Recyclers Try New Ways to Process Spent Catalyst," Chemical Engineering, Feb. 16, 1987, pp. 25-31. This reference discloses a Gulf Chemical and Metallurgical Corporation metal recovery method wherein molybdenum and vanadium are converted to their sodium salts by adding sodium carbonate during a multiple-hearth roast at 650.degree.-900.degree. C. This roast removes carbon and sulfur. Calcined material is then quenched in water which dissolves the salts. A leach liquor is then separated from the insolubles by countercurrent decantation. Later, ammonium chloride is added to precipitate ammonium vanadate, which is calcined and fused to produce vanadium pentoxide. The remaining solution is then heated to 80.degree.-85.degree. C. and acidified to precipitate molybdic acid, which is calcined to molybdic oxide. Other examples of metal recovery techniques are disclosed in Trimm, D. L., "Deactivation, Regeneration, and Disposal of Hydroprocessing Catalysts," Catalyst in Petroleum Refining 1989, Elsevier Science Publishers, B. V. Amsterdam (1990).
Catalyst rejuvenation involves selective removal of contaminant metals from the spent catalyst followed by or preceded by oxidative decoking of the coke deposited on the catalyst. The objective in catalyst rejuvenation is to remove the contaminant metals while leaving the catalytic metals and to reuse the catalyst rather than to reclaim the metals or dispose of the catalyst entirely. One way of achieving selective removal of contaminant metals, in particular vanadium, is by attrition. It is well known that vanadium and iron are deposited on the exterior of some catalyst. As a result, subjecting the outermost layers of the catalyst to abrasion leads to a powder rich in coke, vanadium and iron, while the residual pellet contains catalytic material and nickel (which is deposited throughout the pellet). Although some improvement in activity is observed, the overall benefit is not high because abrasion of the exterior deposits still leaves vanadium that is deposited in the pore mouths of the catalyst. Deeper abrasion to remove pore-mouth deposits can weaken the catalyst. Another approach for selectively removing contaminant metals from the spent catalyst is selective leaching which takes advantage of the fact that metals on the spent catalyst are usually present as sulfides. Selective leaching involves treating the spent hydroprocessing catalyst with a chemical that reacts with only one of these sulfides, e.g. the use of oxalic acid to remove vanadium. An extensive discussion of the use of leaching to selectively remove metals from spent hydroprocessing catalyst can be found in M. Marafi, A. Stanislaus, C. J. Mumford, and M. Fahim, "Regeneration of Spent Hydroprocessing Catalyst: Metal Removal," Applied Catalysis, 47(1989) pp. 85-96.
The problem with these so-called selective removal processes is, invariably, some of the catalytic metals are also removed, thereby requiring that at least a portion of the catalytic metals be reincorporated onto the catalyst prior to reuse. The problems associated with selective removal of contaminant metals have lead some operators in the industry to catalytically regenerate the spent catalyst. In other words, the spent, metal-containing hydroprocessing catalyst is decoked using oxidative combustion, but the contaminant and catalyst metals are left behind until the metals build-up is so severe that the catalyst needs to be replaced. Regeneration takes into account that deactivation due to coking occurs much more rapidly than deactivation due to metal deposition. In addition, the catalyst's demetallation capacity is more fully utilized, thereby increasing its eventual reclamation value.
The problem with decoking in the presence of contaminant metals, in particular vanadium, is that the catalyst becomes soft, i.e., the catalyst's resistance to attrition is significantly reduced. This problem is particularly severe when the catalyst is employed in an ebullated bed where the solid catalyst particles are kept in random motion by the upward flow of liquid and gas. This random motion makes the attrition resistance of the catalyst a very important property. It is believed that when a spent, metal-containing, hydroprocessing catalyst is subjected to oxidative combustion, one of the species oxidized is the V.sub.3 S.sub.4 sulfide, the predominant vanadium phase deposited under typical hydrotreating conditions. The sulfide is then converted to vanadium pentoxide (V.sub.2 O.sub.5). Water formed during the combustion step reacts with the pentoxide to form vanadic acid, VO(OH).sub.3, a volatile and highly reactive species that reacts with metals present in the catalyst such as iron, nickel, aluminum or molybdenum to form mixed metal vanadates. These vanadates cause loss of both catalyst surface area and attrition resistance.
The use of passivating agents such as tin and titanium to reduce the detrimental effects of contaminant metal-containing hydrocarbon feedstocks on cracking catalyst is well known in the art. U.S. Pat. Nos. 4,326,990 and 4,255,287 disclose contacting a cracking catalyst with an agent containing tin and antimony prior to contacting the catalyst with a contaminant metal-containing hydrocarbon feedstock. U.S. Pat. No. 4,324,648 discloses treating a cracking catalyst with agent containing tin, phosphorous, and sulfur prior to contacting the catalyst with a contaminant metal-containing hydrocarbon feedstock. U.S. Pat. No. 2,886,513 is directed to employing titanium chloride togethre with calcium oxide to obtain a catalytic cracking catalyst which is effective to produce a more olefinic product than other catalytic cracking catalysts. U.S. Pat. No. 4,496,665 discloses continuous addition of a titanium additive to a cracking catalyst employed in conversion of contaminant metal-containing hydrocarbon feedstocks to promote matrix cracking of large molecules. U.S. Pat. No. 3,696,025 discloses adding titanium to a cracking catalyst during the catalytic cracking process and subsequently regenerating the cracking catalyst to increase the CO.sub.2 /CO ratio in the regeneration effluent.
Since the early 1960's, cracking catalysts have predominately consisted of molecular sieves incorporated into an amorphous matrix of silica, alumina, silica-alumina, kalin, clay or the like. Such catalysts were 1,000-10,000 times more active than previous amorphous silica-alumina catalysts due to their acidity. These molecular sieve-containing catalysts are not suitable for use in resid hydrotreating because their high activities promote coke formation and their relative high micropore volumes promote pore mouth plugging of the catalyst.